When you think about it, it's amazing how many different types of electronic memory you encounter in daily life. Many of them have become an integral part of our vocabulary.
What is computer memory explain? What is the role of memory in computer functioning? How is computer memory measured? What are the two types of primary memory? type of computer memory what is memory? how data is stored in computer memory ? computer memory units ? how rom works how ram works What are the two types of primary memory?
What you may not know is that most of the electronic items you use everyday have some form of memory also. here are just a few examples of the many item that use memory:
What is computer memory explain? What is the role of memory in computer functioning? How is computer memory measured? What are the two types of primary memory? type of computer memory what is memory? how data is stored in computer memory ? computer memory units ? how rom works how ram works What are the two types of primary memory?
- RAM
- ROM
- Cache
- Dynamic RAM
- Static RAM
- Flash memory
- Memory Sticks
- Virtual memory
- Video memory
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What you may not know is that most of the electronic items you use everyday have some form of memory also. here are just a few examples of the many item that use memory:
- Cell Phone
- PDAs
- Game consoles
- Car Radios
- VCRs
- TVs
Memory Basics
Although Memory is technically any form of electronic storage, it is used most often to identify fast, temporary forms of storage. if your computer's CPU had to constantly access the hard drive to retrieve every piece of data it needs, it would operate very slowly. When the information is kept in memory, the CPU can access it much more quickly, most forms of memory are intended to store data temporarily. As you can see in the diagram on the left side, the CPU accesses memory according to a distinct hierarchy. Whether it comes from permanent storage (the hard drive) or input (the keyboard ), most data goes in random access memory (RAM ) first. The CPU then stores pieces of data it will need to access, often in a cache, and maintains certain special instruction in this register.
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The PC process
All of the components in your computer, such as the CPU , the hard disk and the operating system, work together as a team, and memory is one of the most essential parts of this team. From the moment you turn your computer on until the time you shot it down, your CPU is constantly using memory. Le's take a look at a typical scenario:
You turn your computer on. The computer loads data from read-only memory (ROM) and performs a power-on self-test (POST) to make sure all major components are functioning properly. As part of this test, the memory controller checks all of the memory addresses with a quick read/write operating to ensure that there are no errors in the memory chips read/write means that data is written to a bit and then read from that bit the computer loads the basic input/output system (BIOS) from ROM the BIOS provides the most basic information about storage devices, boot sequence, security, plug and play(auto device recognition )capability and a few other item.
The computer loads the operating system (OS) From the hard drive into the system's RAM. Generally, the critical part of the operating system are maintained in RAM as long as the computer is on. This allows the CPU to have immediate access to the operating system, Which enhances the performance and functionality of the overall system.
When you open an application, it is loaded into RAM . to conserve RAM Usage, many applications load only the essential parts of the program initially and then loads other pieces as needed. After an application is loaded, any files that are opened for use in that application are loaded into RAM
When you save a file and close the application, the file is written to the specified storage device, and then the application are purged from RAM . in the list above, every time something is loaded or opened, it is placed into RAM . this simply means that it has been put in the computer's temporary storage area so that the CPU can access that information more easily . the CPU requests the data it needs from RAM , processes it and writes new data back to RAM in a continuous cycle. in most computers, this shuffling of data between the CPU and RAM happens millions of times every second. when an application is closed, it and any accompanying files are usually purged(deleted) from RAM to make room for new data. if the changed files are not saved to a permanent storage device before being purged, they are lost.
The Need for Speed
One common question about desktop computers that comes up all the time is "Why does a computer need so many memory system?" a typical computer has:
Level 1 and Level 2 caches
Normal system RAM
Virtual memory
A hard disk
Fast. powerful CPU s need quick and easy access to large amounts of data in order to maximize their performance. in the CPU cannot get to the data it needs, it literally stops and waits for it modern CPUs running at speeds of about 1 gigahertz can consume massive amounts of data- potentially billions of bites per second. the problem that computer designers face is that memory that can keep up with a 1-gigahertz CPU is extremely expensive-much more expensive than anyone can afford in large quantities.
Memory tiers
Computer designers have solved the cost problem by "tiering" memory- using expensive memory in small quantities and then backing it up with larger quantities of less expensive memory. the cheapest form of read/write memory in wide use today use the hard disk. hard disks provide large quantities of expensive, permanent storage. you can buy hard disk space for pennies per megabyte, but it can take a good bit of time (approaching a second) to read a megabyte off a hard disk. because storage space on a hard disk is so cheap and plentiful, it forms the final stage of a CPUs memory hierarchy, called virtual memory. the next level of the hierarchy is RAM , but several points about RAM are important here.
The bit size of a CPU tells you how many bytes of information it can access from RAM at the same time. for example, a 16-bit CPU can process 2 bytes at a time (1 byte = 8 bits, so 16 bits = 2 bytes), and a 64-bit CPU can process 8 bytes at a time.
Megahertz (MHz) is a measure of a CPU's processing speed, or clock cycle, in millions per second. so, a 32-bit 800-MHz pentium III can potentially process 4 bytes simultaneously, 800 million times per second (possibly more based on pipelining)! the goal of the memory system is to meet those requirements.
A compute's system RAM alone is not fast enough to match the speed of the CPU . that is why you need a cache (discussed later). However, the faster RAM is, the better. most chip today operate with a cycle rate of 50 to 70 nanoseconds. the read/write speed is typically a function of the type of RAM used, such as DRAM,SDRAM,RAMBUS. WE WILL TALK ABOUT THESE VARIOUS TYPES OF MEMORY LATER.
System RAM
System RAM speed is controlled by bus width and bus speed. bus width refers to the number of bits that can't be sent to the CPU simultaneously, and bus speed refers to the number of times a group of bits can be sent each second. a bus cycle occurs every time data travels form memory to the CPU . for example, a 100-MHz 32-bit bus is theoretically capable of sending 4 bytes (32 bits divided by 8= 4 bytes) of data to the CPU 100 million times per second, while a 66-MHz 16-bit bus can send 2 bytes of data 66 million times per second. if you do the math, you'll find that simple changing the bus width from 16 bits to 32 bits and the speed from 66 MHz to 100 MHz in our example allows for three times as much data (400 million bytes versus 132 million bytes) to pass through to the CPU every second.
in reality, RAM doesn't usually operate at optimum speed. Latency changes the question radically. Latency refers to the number of clock cycles needed to read a bit of information. for example, RAM rated at 100 MHz is capable of sending a bit in 0.00000001 seconds, but may take 0.00000005 seconds to start the read process for the first bit. to compensate for latency, CPUs use a special technique called burst mood.
BUrst Moode and Pipelining
Burst mode depends on the expectation that data requested by the CPU will be stored in sequential memory cells. the memory controller anticipates that whatever the CPU is working on will continue to come from this same series of memory addresses, so it reads several consecutive bits of data together. this means that only the first bit is subject to the full effect of latency; reading successive bits takes significantly less time. the rated burst mode of memory is normally expressed as four numbers separated by dashes. the first number tells you the number of clock cycles needed to begin a read operation; the second, third and fourth numbers tell you how in conjunction with pipelining, another means of minimizing the effects of latency. pipelining organizes data retrieval into a sort of assembly-line process. the memory controller simultaneously reads one or more words from memory, sends the current word or words to the CPU and writes one or more words to memory cells. used together, burst mode and pipelining can dramatically reduce the lag caused by latency.
So Why Wouldn't you buy the fastest, widest memory you can get?
The speed and width of the memory's bus should match the system's bus. You can use memory designed to work at 100 MHz in a 66-MHz system, but it will run at the 66-MHz speed of the bus so there is no advantage, and 32-bit memory won't fit on a 16-bit bus.
Cache and Registers
Even with a wide and fast bus, it still takes longer for data to get from the memory card to the CPU than it takes for the CPU to actually process the data. caches are designed to alleviate this bottleneck by making the data used most often by the CPU instantly available.
This is accomplished by building a small amount of memory, know as primary or level 1 cache, right into the CPU. Level 1 cache is very small, normally ranging between 2 kilobytes (KB) and 64 KB.
The secondary or level 2 cache typically resides on a memory card located near the CPU the level 2 cache has a direct connection to the CPU . A dedicated integrated circuit on the motherboard, the L2 controller, regulates the use of level 2 cache by the CPU . Depending on the CPU . the size of the level 2 cache ranges from 256 KB to 2 megabytes (MB). In most systems, data needed by the CPU is accessed from the cache approximately 95 percent of the time, greatly reducing the overhead needed when the CPU has to wait for data form the main memory.
some inexpensive systems dispense with the level 2 cache altogether. many high performance CPUs now have the level 2 cache actually built into the CPU chip itself. therefore, the size of the level 2 cache and whether it is onboard (on the CPU ) is a major determining factor in the performance of a CPU.
A particular type of RAM, static random access memory (SRAM), is used primarily for cache. SRAM use multiple transistor, typically four to six, for each memory cell. it has an external gate array know as a bistable multivibrator that switches, or flip-flops, between two states. this means that it does not have to be continually refreshed like DRAM. each cell will maintain its data as long as it has power. without the need for constant refreshing SRAM can operate extremely quickly. but the complexity of each cell makes it prohibitively expensive for use as standard RAM.
The SRAM in the cache can be asynchronous. synchronous SRAM is designed to exactly match the speed of the CPU , while asynchronous is not. that tittle bit of timing makes a difference in performance. matching the CPU's clock speed is a good thing, so always look for synchronized SRAM.
The final step in memory is the registers. these are memory cells built right into the CPU that contain specific data needed by the CPU , particularly the arithmetic and logic unit (ALU).An integral parl of the CPU itself, they are controlled directly by the compiler that sends information for the CPU to process.
Type of Memory
Memory can be spilt into two main categories: volatile and nonvolatile. Volatile ,memory loser any data as soon as the system is turned off: it requires constant power to remain viable. most types of RAM fall into this category. nonvolatile memory does not loss its data when the system or device is turned off. A number of types of memory fall in to this category. the most familiar is ROM but flash memory storage devices such as compactflash or smartmedia cards are also forms of no-volatile memory
How RAM Works
Random access memory (RAM) is the best known form of computer memory. RAM is considered "random access" because you can access any memory cell directly if you know the row and column that interest at that cell. the opposite of RAM is serial access memory (SRAM). SAM stores data as a series of memory cells that can only be accessed sequentially (like a cassette tape). if the data is not in the current location, each memory cell is checked until the needed data is found. SAM works very well for memory buffers, where the data is normally storeddd in the order in which it will be used (a good example is the texture buffer memory on a video card). RAM data, on the other hand, can be access in any order.
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RAM Basics
Similarly to a microprocessor, a memory chip is an integrated circuit (IC)made of millions of transistors and capacitors. in the most common form of computer memory, dynamic random access memory (DRAM), a transistor and a capacitor are paired to create a memory cell, which represents a single bit of data. the capacitor holds the bit of information-a 0 or a. the transistor acts as a switch that lets the control circuitry on the memory chip read the capacitor or change its state.
A capacitor is like small bucket that is able to store electrons. to store a 1 in the memory cell, the bucket is filled with electrons. to store a 0, it is emptied. the problem with the capacitors bucket is that it has a leak. in a matter of a few milliseconds a full bucket becomes empty. therefore, for dynamic memory to work, either the CPU or the memory controller has to come along and recharge all of the capacitors holding a 1 before they discharge. to do this the memory controller reads the memory and then writes it right back this refresh operation happens automatically thousands of times per second.
the capacitor in a dynamic RAM memory cell is like a leak bucket. it needs to be refreshed periodically or it will discharge to 0.
this refresh operation is where dynamic RAM gets its name. dynamic RAM has to be dynamically refreshed all of the time or it forgets what is holding. the downside of all of this refreshing is that it takes time and slow down the memory. memory cells are etched onto a silicon wafer in an array of columns(bitlines)and rows (wordlines). the intersection of a bitlines and wordline constitutes the address of the memory cell.
DRAM
DRAM works by sending a charge through the appropriate column (CAS)to activate the transistor at reach bit in the column. when writing, the row lines contain the state the capacitor should take on. when reading, the sense-amplifier determines the level of charge in the capacitor. if it is more than 50 percent, it reads it as a 1; otherwise it reads it as a 0. the counter tracks the refresh sequence based on which rows have been accessed in what order. the length of time necessary to do all this is so short that it is expressed in nanoseconds (billionths of a second).a memory chip rating of 70ns means that it takes 70 nanoseconds to completely read and recharge each cell.
memory cells alone would be worthless without some way to get information in and out of them. so the memory cells have a whole support infrastructure of other specialized circuits. these circuits perform functions such as:
Identifying each row and column (row address select and column address select)
keeping track of the refresh sequence (counter)
reading and restoring the signal from a cell(sense amplifier)
telling a cell whether it should take a charge or not (write enable)
static ram
use a completely different technology. in static RAM , a form of flip-flop holds each bit of memory. a flip-flop for a memory cell takes four or six transistors along with some wiring, but never has to be refreshed. this makes statics RAM significantly faster than dynamic RAM . However, because it has more parts, a static memory cell takes up a lot more space on a chip than a dynamic memory cell. therefore, you get less memory per chip and that makes static RAM a lot more expensive.
so static RAM is fast and expensive, and dynamic ram forms the larger system ram space.
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